Interpretive Summary: For successful biocontrol of disease caused by soilborne plant pathogens, an appropriate product containing a biocontrol agent must be developed which can be made commercially and which can be applied to soils infested with the pathogen. In this report, a clay carrier (pyrax) was mixed with fermentor-produced biomass of isolate Gl-3 of Gliocladium virens, and applied at four rates (15, 30, 60, 120 g/l.1 m*2) to field plots infested with the pathogen Sclerotium rolfsii in a 2-year study. Plots were planted with snapbeans and damping-off disease assayed at 11 and 35 days after planting. Populations of Gl-3 in the plots were also assayed. In 1992, increased rates of the product reduced disease so that rates of 60 and 120 g per subplot resulted in plant stands comparable to those in the noninfested plots. In 1993, all rates reduced disease incidence but stands were not as high as those in noninfested soils. Populations of Gl-3 increased in the soil subplots which suggested that the biocontrol agent can become established in soil. In greenhouse trials to evaluate the effect of formulations with biomass of 15 isolates of Trichoderma spp. and G. virens on sclerotial germination of strains Sr-1 and Sr-3 of S. rolfsii, isolates of G. virens were more effective than isolates of Trichoderma spp. in preventing sclerotial germination. These results demonstrate the feasibility of this product to control diseases caused by S. rolfsii. It benefits society because it eliminates the need for fungicides which may enter the food chain as well as cause environmental pollution. The product can also be easily prepared by industry and readily applied by growers.

Technical Abstract:
A 2-year field study at Beltsville, MD in soil artificially infested with potato-dextrose agar grown sclerotia of strain Sr-1 of Sclerotium rolfsii demonstrated the ability of fermentor-produced biomass of isolate Gl-3 of Gliocladium virens in a powder preparation to prevent damping-off of snap beans caused by the pathogen. Plant stands were counted 11 and 35 days after seed planting and the colony-forming units (cfu)/g of soil of Gl-3 i the treatment subplots were determined. Pyrax/biomass amended at rates of 15, 30, 60, and 120 g/1.1 m*2 of subplot significantly increased plant stands after 35 days more than the stand in the pathogen infested control soils for 1992 and 1993, respectively. In 1992, the stand increase was correlated with increased rates of the preparation, such that 60 and 120 g of the Pyrax/biomass per subplot resulted in stands comparable to that (>85%) in the noninfested control plots. In 1993, there was no correlation nbetween rate of amendment and plant stand and all rates gave stands greate than that in the infested control but not as high as that in the noninfested controls. Generally, populations of Gl-3 increased by 11 days with higher rather than lower rates of Pyrax/biomass. Population levels declined after 35 days of plant growth. In a study to determine the influence of Pyrax/biomass of various isolates of Trichoderma spp. and G. virens on germination of sclerotia of two strains (Sr-1 and Sr-3) of S. rolfsii, there was considerable specificity. Isolates of G. virens were more effective in reducing sclerotial germination than were isolates of T. viride, T. hamatum and T. harzianum. Strain Sr-3 was less affected by the various biocontrol isolates than was Sr-1.